Why does it render smooth?
I know I did by mistake a key combination in sculpt mode....


From looking at your image, and from your description, it sounds like you most likely pressed Ctrl+2 or otherwise activated the Multi-resolution Modifier while sculpting.

This modifier has separate preview/ sculpt and render settings much like the Subdivision Surface Modifier as seen in the left column of the modifier settings. 



From the manual:


  Levels
  
  The left column of settings control the general quality generated by
  the modifier in various contexts.
  
  Preview
  
  Set the level of subdivisions to show in Object Mode. Sculpt
  
  Set the level of subdivisions to use in Sculpt Mode. Render
  
  Set the level of subdivisions to show when rendering. Quality
  
  How precisely the vertices are positioned (relatively to their theoretical position), can be lowered to get a better performance when
  working on heavy meshes.


If the render and preview amounts do not match, you will see a different result when rendering, although performance may be improved with lower sculpt/ preview amounts in some cases. 

In your specific situation however, needing a low-poly object you can simply remove the modifier. (click the x icon)